1. Field of the Invention
The invention relates to a technology concerning a laser welding apparatus and a laser welding method.
2. Description of Related Art
The laser welding is a welding method in which laser generated by a laser oscillator is converged by a condenser lens, and is applied to superposed steel sheets so as to weld the sheets. However, in the laser welding, the joint strength declines in the case where the sheet gap between steel sheets that are superposed together is large. For example, if there is an inter-sheet gap larger than or equal to 0.3 mm, the weld bead surface sometimes sinks, resulting in a reduced joint strength. Besides, if the inter-sheet gap is larger than 0.5 mm, it sometime happens that a penetration hole forms and the joint strength declines.
In the welding of a three-dimensional shape object, for example, a motor vehicle body, etc., it is difficult to properly control the inter-sheet gap to, for example, 0.3 mm or less. Meanwhile, an inter-sheet gap of about 1 mm is permitted practically, in the case of manufacturing of motor vehicle bodies and the like. Therefore, it has become an issue to weld steel sheets with an inter-sheet gap of about 1 mm without a decline in the joint strength. Now, some measures against the reduction of the joint strength of the laser welding of a large inter-sheet gap are taken in practice.
Japanese Patent Application Publication No. 2004-025219 (JP 2004-025219 A) discloses a method in which an inter-sheet gap is welded by using a shim roller. However, in the case where a motor vehicle body or the like is welded, two or three steel sheets are sometimes welded. Therefore, in the welding method disclosed by JP 2004-025219 A, there is a need to highly frequently carry out the replacement of shims or the like, and therefore the production efficiency declines.
Japanese Patent Application Publication No. 2005-131707 (JP 2005-131707 A) discloses a method in which laser tack welding is performed alternately with welding sites with the use of a clamp. However, in a method disclosed by JP 2005-131707 A, a dedicated large-size clamp is needed for the welding of a motor vehicle body or the like. Besides, in the welding method disclosed in the JP 2005-131707 A, when a zinc-plated steel is to be welded, the laser tack welding results in a welding failure caused by plating vapor blow.
In Japanese Patent Application Publication No. 2010-023047 (JP 2010-023047 A), the first laser irradiation is performed in a defocused state to melt the upper sheet so that a protrusion is formed toward the lower sheet side and therefore the gap is reduced, and then the second laser irradiation is performed to accomplish a penetrating weld. However, according to the welding method disclosed in the JP 2010-023047 A, since the first laser irradiation is low-energy irradiation, the first laser irradiation achieves the heat conduction melting in the upper sheet instead of the keyhole melting. Therefore, the processing time becomes long, and the productivity declines.